Re: why my systems never cache more than ~900 MB?

[Nick Piggin <nickpiggin@yahoo.com.au>]

On Tuesday 24 March 2009 19:42:08 Tomasz Chmielewski wrote:
> On my (32 bit) systems with more than 1 GB memory it is impossible to cache
> more than about 900 MB. Why?
>
> Caching never goes beyond ~900 MB (i.e. when I read a mounted drive with
> dd):

Because blockdev mappings are limited to lowmem due to sharing their
cache with filesystem metadata cache, which needs kernel mapped memory.
It will >900MB of pagecache data OK (data from regular files)

> # free
>              total       used       free     shared    buffers     cached
> Mem:       2076164     966788    1109376          0     855132      68932
> -/+ buffers/cache:      42724    2033440
> Swap:      2097144          0    2097144
>
>
> Same behaviour on 32 bit machines with 4 GB RAM.
>
> No problems on 64 bit machines.
> I have one 32 bit machine that caches beyond ~900 MB without problems.

Does it have a different user/kernel split?


> Is it some kernel/proc/sys setting that I'm missing?

No, it just can't be done without changing code.

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